Wednesday, November 29, 2017

The Widespread Impact of Bursting Bubbles

What really determines clouds and rain? Why does burning rubber smoke so heavily? What gives sparkling wines their distinct and lively aroma? The answer to all of these questions leads back to bubbles, according to Alfonso Gañán-Calvo from the University of Seville in Spain. When small bubbles on the surface of a liquid burst, they often send tiny droplets of the liquid flying into the air. These tiny droplets spread out through the air and, if they contain solutes or particles, the relics after liquid evaporation become seeds for clouds, far-reaching scents, or heavy smoke.

In research recently published in the American Physical Society’s journal Physical Review Letters, Gañán-Calvo determined the relationship between the size and speed of the ejected droplets, and the properties of the liquid and the size of the bursting bubble. This knowledge could help scientists working in areas ranging from geophysics to the food industry, including those studying the impact of humans on the environment.

When bubbles burst on the sea surface, marine aerosols—atoms and molecules commonly found in saltwater like sodium chloride, potassium, magnesium, and others—spread out into the atmosphere and become seeds around which clouds can form. Aerosols are released in many other processes too, like heating oil and burning biomass. This has consequences—for example, as we highlighted in a recent Physics Buzz story, evidence suggests that the aerosols in ship exhaust lead to more intense storms over busy shipping routes. Aerosols impact water patterns, the amount of energy that reaches the earth from the sun, and air quality.

Crash into me
Image Credit: Tisha M. via Flickr. (CC BY 2.0)

Earlier this year, Gañán-Calvo was part of a team working on the technology behind creating tiny drops of liquid on demand. He noticed a correlation between the size and speed of ejected droplets and was inspired to look at the existing research behind the somehow related phenomenon of bubble bursting. It turned out that this bubble bursting process had been studied in many different contexts, but no one had pulled together all of the data to explore a universal solution to the question of what determines the size and speed of ejected droplets.

With a background in aerosols and a long-standing concern about issues of climate change, global warming, and human impact on the planet, Gañán-Calvo took on the challenge. “The fact that an enormous source of these aerosols on Earth came from the very same and unique phenomenon pushed me to focus all my efforts on this problem,” he says.

His work focused on modeling the physics of the situation—from the bubble bursting to the first droplet of liquid that goes flying into the air. When the conditions are right, meaning that the bubble is large enough (from tens of microns to millimeters) or the liquid has a low viscosity, the bubble’s collapse will cause a cavity in the water and the formation of two jets. Like the splash that occurs when you drop a rock into a pool, one jet shoots upward and may eject droplets into the air. Simultaneously, the other jet travels in the opposite direction, underwater, and adds momentum and sometimes more gas bubbles to the liquid underneath the cavity.

Drops #2 shows an upward-traveling water jet.
Image Credit: David Bestivo (CC BY-SA 2.0)

As all of this happens, the water flows in complex but repeatable patterns, says Gañán-Calvo. By analyzing the physics of liquid particles on the surface of the liquid close to where the jets form, he found relationships between the initial velocity of the jet, the radius of the first emitted droplet, the size of the bubble, and the density, viscosity, and surface tension of the liquid. He also determined how much kinetic energy is required for a droplet to be ejected at all, and under what conditions this occurs.

To test the validity of his model, Gañán-Calvo went back in time. He found two hundred measurements of this phenomena that included bubble size and droplet size, some dating back as early as 1954. He compared this data to his proposed model and found consistent, excellent agreement. The new model shows that under the right conditions, droplets can exceed supersonic speeds and reach great distances.

This work came together over just a few months and the result is widely applicable to many areas, including climate change. Human impact and pollution on coastal zones and in other critical sea regions causes changes in the surface properties of seawater and consequently on marine aerosol distributions, impacting cloud formation, cloud activity, radiation balance, and precipitation. For instance, many modern oil-spill cleanup methods rely on changing the surface tension of the water, which could have significant impacts on droplet formation. Understanding the relationship between aerosols and bursting bubbles is crucial to understanding and mitigating the extent of our influence. “This may be one of my best papers, being conceived and written in the shortest timeframe of my life,” says Gañán-Calvo.

Kendra Redmond

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